Different retransmission schemes have been developed for correcting transmission errors on a wireless link between a base station and a mobile station.
Using an automatic repeat request (ARQ=automatic repeat request) scheme, a transmitter adds cyclic redundancy check information (CRC=cyclic redundancy check) to data to be transmitted. A receiver can use the CRC information to detect an error in the received data. Depending on a detection of the error, the receiver sends a positive acknowledgement (ACK=acknowledgement) or a negative acknowledgement (NACK=negative acknowledgement) to the transmitter. The receiver does not store received data, which cannot be received error free. If the transmitter receives a NACK, the transmitter retransmits the data at least a second time.
A hybrid ARQ scheme (HARQ=hybrid ARQ) combines soft combining with the ARQ scheme. The soft combining avoids a discarding of an erroneously received data packet by storing the erroneously received data packet in a data buffer of the receiver. This allows retransmitting either the whole data packet in a same way with all information bits (denoted as chase combining) and with redundancy information by adding parity bits. The parity bits are calculated from the information bits using a specific coding scheme and a specific rate matching. Alternatively besides the parity bits, only a subset of the information bits or none of the information bits (denoted as incremental redundancy) are retransmitted.
A HARQ process comprises a first transmission of data and either a positive acknowledgement or one or more retransmissions of parity information for the data, if the first transmission is not acknowledged until one of the retransmissions is acknowledged or until a predefined number of retransmissions have been performed.
Usually, two or more HARQ processes are running simultaneously and each of the two or more HARQ process is related to transmitting and acknowledging a single protocol data unit. An identifier may be used for assigning positive and negative acknowledgements to the two or more HARQ processes. For instance, in LTE up to eight HARQ processes are running simultaneously on a terminal. Consequently, eight different identifiers of the HARQ processes are required. An identifier of a HARQ process may comprise an integer value, preferably in the range from 0 to 7.
In a cooperative uplink relaying scheme, a base station incorrectly receives a first data packet from a first mobile station and incorrectly receives a second data packet from a second mobile station, while a relay node between the mobile station and the base station correctly receives the first and the second data packet. The relay node performs network coding by combining a retransmission of the first data packet for the first mobile station and a retransmission of the second data packet for the second mobile station and transmits the combined retransmission to the base station.
In a multiuser ARQ scheme, a base station transmits a first data packet to a first mobile station and a second data packet to a second mobile station. The first mobile station may receive the first data packet incorrectly but may receive the second data packet correctly. Similarly, the second mobile station may receive the second data packet incorrectly but may receive the first data packet correctly. The base station performs network coding by combining a retransmission of the first data packet for the first mobile station and a retransmission of the second data packet for the second mobile station and multicasts or broadcasts the combined retransmission to the first and the second mobile station.
For the multiuser ARQ scheme, the first mobile station wastes electrical power, because it must also decode the second data packet destined for the second mobile station. Likewise, the second mobile station wastes electrical power, because it must also decode the first data packet destined for the first mobile station.
Furthermore, the multiuser ARQ scheme can be only applied, if the first mobile station correctly receives the second data packet and the combined retransmission of the first and the second data packet and if the second mobile station correctly receives the first data packet and the combined retransmission of the first and the second data packet.
The cooperative uplink relaying scheme and the multiuser ARQ scheme require data packets from or for at least two mobile stations. The radio conditions or the distribution of the mobile stations across a coverage area of a base station may not allow applying the cooperative uplink relaying scheme or the multiuser ARQ scheme all the time. Thereby, an amount of signaling data or an amount of user data for single retransmissions of user data may consume additional radio resources.